Merge branch 'main' into refactor-docstring-params

docs/sphinx/source/whatsnew/v0.10.0.rst

@@ -49,6 +49,9 @@ Enhancements
 Bug fixes
 ~~~~~~~~~
 
+* Prevent small negative values of `v_oc` in :py:func:`pvlib.singlediode._lambertw`
+  which result from accumulated roundoff error. (:issue:`1780`, :issue:`1673`, :pull:`1782`)
+
 
 Testing
 ~~~~~~~
@@ -71,3 +74,5 @@ Contributors
 * Adam R. Jensen (:ghuser:`AdamRJensen`)
 * Echedey Luis (:ghuser:`echedey-ls`)
 * Cliff Hansen (:ghuser:`cwhanse`)
+* Cédric Leroy (:ghuser:`cedricleroy`)
+* Jean-Baptiste Pasquier (:ghuser:`pasquierjb`)

pvlib/inverter.py

@@ -335,7 +335,7 @@ def pvwatts(pdc, pdc0, eta_inv_nom=0.96, eta_inv_ref=0.9637):
     NREL's PVWatts inverter model.
 
     The PVWatts inverter model [1]_ calculates inverter efficiency :math:`\eta`
-    as a function of input DC power
+    as a function of input DC power :math:`P_{dc}`
 
     .. math::
 
@@ -369,6 +369,10 @@ def pvwatts(pdc, pdc0, eta_inv_nom=0.96, eta_inv_ref=0.9637):
 
     Notes
     -----
+    When sourcing ``pdc`` from pvlib functions
+    (e.g. :py:func:`pvlib.pvsystem.pvwatts_dc`) their DC power output is in W,
+    and ``pdc0`` should have the same unit (W).
+
     Note that ``pdc0`` is also used as a symbol in
     :py:func:`pvlib.pvsystem.pvwatts_dc`. ``pdc0`` in this function refers to
     the DC power input limit of the inverter. ``pdc0`` in
@@ -393,6 +397,7 @@ def pvwatts(pdc, pdc0, eta_inv_nom=0.96, eta_inv_ref=0.9637):
     pdc_neq_0 = ~np.equal(pdc, 0)
 
     # eta < 0 if zeta < 0.006. power_ac is forced to be >= 0 below. GH 541
+    # In some published versions of [1] the parentheses are missing
     eta = eta_inv_nom / eta_inv_ref * (
         -0.0162 * zeta - np.divide(0.0059, zeta, out=eta, where=pdc_neq_0)
         + 0.9858)  # noQA: W503

pvlib/pvsystem.py

@@ -2809,9 +2809,9 @@ def pvwatts_dc(g_poa_effective, temp_cell, pdc0, gamma_pdc, temp_ref=25.):
 
         P_{dc} = \frac{G_{poa eff}}{1000} P_{dc0} ( 1 + \gamma_{pdc} (T_{cell} - T_{ref}))
 
-    Note that the pdc0 is also used as a symbol in
-    :py:func:`pvlib.inverter.pvwatts`. pdc0 in this function refers to the DC
-    power of the modules at reference conditions. pdc0 in
+    Note that ``pdc0`` is also used as a symbol in
+    :py:func:`pvlib.inverter.pvwatts`. ``pdc0`` in this function refers to the DC
+    power of the modules at reference conditions. ``pdc0`` in
     :py:func:`pvlib.inverter.pvwatts` refers to the DC power input limit of
     the inverter.
 
@@ -2836,7 +2836,7 @@ def pvwatts_dc(g_poa_effective, temp_cell, pdc0, gamma_pdc, temp_ref=25.):
     Returns
     -------
     pdc : numeric
-        DC power.
+        DC power. [W]
 
     References
     ----------

pvlib/singlediode.py

@@ -794,6 +794,13 @@ def _lambertw(photocurrent, saturation_current, resistance_series,
     # Compute open circuit voltage
     v_oc = _lambertw_v_from_i(0., **params)
 
+    # Set small elements <0 in v_oc to 0
+    if isinstance(v_oc, np.ndarray):
+        v_oc[(v_oc < 0) & (v_oc > -1e-12)] = 0.
+    elif isinstance(v_oc, (float, int)):
+        if v_oc < 0 and v_oc > -1e-12:
+            v_oc = 0.
+
     # Find the voltage, v_mp, where the power is maximized.
     # Start the golden section search at v_oc * 1.14
     p_mp, v_mp = _golden_sect_DataFrame(params, 0., v_oc * 1.14, _pwr_optfcn)

pvlib/tests/test_singlediode.py

@@ -168,6 +168,19 @@ def test_singlediode_precision(method, precise_iv_curves):
     assert np.allclose(pc['i_xx'], outs['i_xx'], atol=1e-6, rtol=0)
 
 
+def test_singlediode_lambert_negative_voc():
+
+    # Those values result in a negative v_oc out of `_lambertw_v_from_i`
+    x = np.array([0., 1.480501e-11, 0.178, 8000., 1.797559])
+    outs = pvsystem.singlediode(*x, method='lambertw')
+    assert outs['v_oc'] == 0
+
+    # Testing for an array
+    x  = np.array([x, x]).T
+    outs = pvsystem.singlediode(*x, method='lambertw')
+    assert np.array_equal(outs['v_oc'], [0, 0])
+
+
 @pytest.mark.parametrize('method', ['lambertw'])
 def test_ivcurve_pnts_precision(method, precise_iv_curves):
     """